In recent years, the regulatory environment in all aspects of Environment, Safety and Health (ESH) has been highlighted on a global scale. Incidentally, the term “regulatory environment” used here is essentially different from the more local problem of pollution generally, which is relatively local and on a scale such that it is within the earth's ability to self-clean, but still needs some countermeasures to assist it on a global scale.
In the semiconductor industry, environmental management is important as well, and at present, it is very important to reduce perfluorocarbon (PFC) emissions into the environment. In the semiconductor industry, however, the environmental problem is not only that just discussed above, but also that of reducing and recycling waste of acids and organic solvents, and reducing electric power consumption.
In a manufacturing process of semiconductor devices, cleaning up various types of contamination of semiconductor wafers has been carried out by a method where the wafer is dipped in an acidic or alkaline solution, such as a mixed solution of sulfuric acid/hydrogen peroxide, a mixed solution of hydrochloric acid/hydrogen peroxide, and a mixed solution of ammonia/hydrogen peroxide, followed by wafer heating or application of ultrasonic vibration. For example, removal of metal contamination adhered to the wafer surface is carried out by oxidation (ionization) of metal using sulfuric acid or the like, so that the contamination is eluted into a solvent to make into a solvated (hydrated) ion for stabilization.
However, when the waste from this type of cleaning treatment is made nontoxic, waste such as sludge is produced. In addition, there is a large amount of waste liquid produced by the above cleaning treatment and a large amount of electric power and water are necessary for waste treatment. Therefore, cleaning using sulfuric acid or the like has a very large environmental load.
Because of those reasons, it is preferred that water be used as a solvent for a solution for cleaning of wafers. It is also preferred to use a solution containing no elements other than H and O, such as pure water or a hydrogen peroxide solution, instead of an acidic or alkaline solution. Thus, for removal of metal contamination, it is ideal that metal is efficiently ionized using H2O, H2O2 or the like, and is removed as a hydrated ion. For removal of organic contamination and particles, it is ideal that the organic substance is oxidized and decomposed using H2 O, H2O2, or the like.
As such, the cleaning treatment using H2o, H2O2, or the like is very effective from the viewpoint of waste management. On the contrary, however, a large amount of electric power is needed to purify the feed water. Therefore, there is a desire to reduce the amount of pure water used in the rinse processes. Thus, a dry cleaning technique has been pursued as a substitute for a conventional liquid phase cleaning technique.
Among the cleaning treatments of organic substances, removing resist material requires the largest amount of chemical solution. Furthermore, since it is a liquid phase heating treatment, electricity consumption is relatively high in addition to the already large load to air-conditioning equipment necessary for clean rooms. Therefore, various alternative processes have been studied and, as one of them, a process to remove resist material using aqueous ozone of high concentration has been investigated.
Since aqueous ozone consists of O3 and H2O only, treatment with it is very effective to reduce the environmental load from the viewpoint of waste management. In this process, however, achieving a desired throughput is difficult, for the following reasons.
In a resist removing process using aqueous ozone, the resist removal rate is proportional to the concentration of ozone. Accordingly, in order to make the ozone concentration high for increasing the removal rate, it is necessary to lower the temperature of the aqueous ozone. However, when the temperature of the aqueous ozone is lowered, the reaction rate decreases. Therefore, in the above process, there is an upper limit for the resist removal rate.
In addition, the treatment using ozone has another problem. For example, ozone is explosively decomposed into oxygen and, therefore, it requires careful handling.
As to another cleaning technique utilizing the high oxidizing ability of aqueous ozone, a spin cleaning method of a single-wafer type can be used, where aqueous ozone and diluted hydrofluoric acid are alternately supplied to a wafer. In another method, hydrogen peroxide or ammonia is added to aqueous ozone and then an ultrasonic wave (of an MHz region), is applied to promote the production of OH radicals in the liquid. This cleaning treatment works by improving the oxidative ability of the liquid by the OH radical produced. However, in any of those methods, ozone is used and, therefore, the above-mentioned disadvantage is not yet overcome.
Regarding a cleaning method using no ozone, a method has been reported where an ultrasonic wave (of an MHz region) is applied to dissolved aqueous oxygen or dissolved aqueous hydrogen. This method also intends to improve the oxidative ability by promoting the production of OH radicals in the liquid. Since the dissolved aqueous oxygen and dissolved aqueous hydrogen used in this method are relatively safe, there is no need the same level of careful handling as that required in the case of using ozone. However, when dissolved aqueous oxygen is used, there is an upper limit for the dissolved oxygen concentration. In addition, when dissolved aqueous hydrogen is used, there is a disadvantage in that the hydrogen concentration margin for an optimum cleaning effect is narrow, owing to a competitive reaction between OH radical formation and OH radical deactivation by H radicals.
As a method for cleaning a semiconductor wafer, Japanese Patent Laid-Open Nos. 7869/1993 and 137704/1998 disclose a method using a highly functional cleaning solution prepared by applying microwaves to a chemical solution.
Japanese Patent Laid-Open 7869/1993 discloses a method whereby microwaves are used to irradiate pure water that is in contact with a catalyst consisting of palladium or platinum powder. Then pure water, where a wetting property becomes high, is supplied to a use point for cleaning. Although the microwave excitation lifetime of pure water in a liquid phase is not more than several milliseconds, however, the microwave-irradiated pure water in this method is supplied to the use point after passing through a pipe and then being filtered. Therefore, it is likely that the effect of microwave excitation is already lost at the use point.
In order to improve that, Japanese Patent Laid-Open No. 137704/1998 discloses a direct microwave irradiation application to the cleaning vessel. According to this method, the microwave irradiation excites pure water or a cleaning solution, and the molecular group constituting the pure water or cleaning solution is cleaved into a small size. As a result, surface tension of pure water and the cleaning agent solution at the wafer surface becomes lower, wetting increases, and radicals are generated. Therefore, a cleaning solution having a high chemical reactivity can be permeated into the inner side of the fine pores. In addition, since the liquid temperature can be raised uniformly and within a short time by an induced heating effect, a high reaction rate can be achieved. However, even in this method, a large amount of pure water is still consumed for the cleaning, and a large amount of electric power is needed for its production
Environmental problems concerning the cleaning treatment of semiconductor wafers has been explained above and, as will be mentioned below, there are similar problems in other treatments as well.
For producing relatively thin silicon oxide films and metal oxide films used as gate insulating films or capacitors, or for etching a semiconductor film, metal film or insulating film, oxide species have been used that have a relatively strong oxidizing ability such as oxygen, ozone, di-nitrogen monoxide and nitrogen monoxide. Making a film thinner, and the line width smaller, will be more and more desirable in the future. In order to achieve that together with higher film quality (lower defect density), it is important that the oxidizing species are not supplied solely, but rather that both oxidants and reductants are simultaneously supplied to control the reaction rate. For example, in the case of carrying out the heating treatment of a wafer where metal such as tungsten is exposed, a method has been adopted where a partial pressure ratio of oxygen to steam is regulated so that oxidation of tungsten is prevented.
Usually, such treatments (except for etching) are carried out in a heat-treating furnace, such as an electric furnace or an infrared heating furnace. However, in the furnace, thermal efficiency is poor, consumption of electric power is high, and therefore there is a large environmental load.
Further, in every treatment, it is desired to avoid the use of gases that may cause atmospheric ozone layer depletion and/or global warming (e.g., gases having a large global warming potential (GWP)) as a supplying gas and an exhaust gas. The GWP is a product of the lifetime of the used gas in the atmosphere (which is mostly determined by the reaction rate with OH radicals) and the infrared absorption coefficient of said gas at the air window region (an infrared region of about 8-13 μm wavelength, except the infrared absorption band derived from H2O). Thus, it is not recommended to use gases having an absorption band in the region, except for the infrared absorption band derived from H2O, as a supplying gas and/or an exhaust gas.
As mentioned above, although cleaning with acid or alkali is effective for removing metal contamination, organic contamination, or particles, those methods need a waste liquid processing step having a large environmental load. In a cleaning method using aqueous ozone or a cleaning method using oxygen-dissolved water or hydrogen-dissolved water of which practical use has begun as an alternative cleaning method, solubility of such gases in water is several tens of ppm at best and, therefore, concentration of the resulting oxidizing species is limited by solubility. Therefore, it is difficult to achieve a sufficient throughput. In addition, in a liquid phase cleaning process, including a pure water rinse and a spin cleaning method of a single-wafer type, a large amount of pure water is used as a reaction species or solvent and, therefore, equipment for purifying the feed water on a large scale (with a large environmental load) is necessary. Thus, each and any of the above-mentioned surface treating methods does not have a small environmental load, and does not have a high treating ability.